This invention relates to a method for blackening the surfaces of a body of ferrous metal. The invention is particularly applicable to blackening the surfaces of a mask-frame assembly for a shadow-mask-type color television picture tube.
The mask-frame assembly for a color television picture tube usually comprises an apertured mask welded to a frame or other support, and is mounted in the tube with the mask closely spaced from the viewing screen of the tube. The surfaces of the mask and frame, which are usually of a ferrous metal such as cold-rolled steel, are blackened to reduce or prevent corrosion during the manufacture of the tube, to increase the dissipation of heat generated in the mask and frame during the operation of the tube, and to reduce reflections of visible light during the viewing of video images on the viewing screen of the tube. The blackening is a layer of black iron oxide that is so thin that it does not affect the dimension of the parts.
Several methods of blackening the surfaces of the mask, frame and other ferrous-metal parts with a thin black oxide layer have been suggested. The most common methods include baking the parts at about 600.degree. C. in a wet reducing atmosphere. Other methods include applying a strong oxidizing acid mixture to the surfaces and then, after rinsing, baking the parts at about 400.degree. to 500.degree. C. While these processes do the job, nevertheless, it is desirable to reduce the cost of blackening in terms of reducing material, fuel, handling and capital expenses.
Oxidizing salts, such as sodium nitrate and potassium nitrate, are known to blue or blacken steel surfaces and have been used for this purpose for many years. The accepted practice is to immerse the steel parts to be blackened for several minutes into the fused molten salts or hot concentrated aqueous solutions of the salts. While these treatments are useful for massive parts, they are unsatisfactory for masks which are light in weight and delicate in structure and are easily distorted by the treatment in hot liquids. Also, in the case of the mask-frame assembly, the mass difference between the mask and the frame is so great that gross distortions result when the assembly is immersed in the hot molten salt or hot solution. Except for these problems, the use of such oxidizing salts to blacken the mask and frame is desirable since the required materials are cheap, the required reaction temperatures are lower than those normally used, and there is the possibility of lower handling and capital costs.